Platform independent user_regs_struct defination

[jschwinger233]

Reproduce steps:

1. python3.8 -mhttp.server
2. docker run -it --rm --pid host --net host --privileged python:3 bash
3. python -mpip install git+https://github.com/jschwinger233/pdbattach.git
4. pdbattach $(pgrep -f http.server)
5. inspect the coredump, the data expected to be at $rdi is at $rbp